Wavefronts can contain dislocation lines, closely analogous to those found in imperfect crystals. Surrounding these dislocations, the wave field can have interesting properties. For example, optical vortex (“OV”) beams contain a wavefront phase singularity in their center. Such beams can propagate in free space or in special kinds of optical fiber waveguide. OV beams have a variety of potential applications in many areas, including particle manipulation, micro fabrication, and optical communications. OV beams are characterized by helical phase fronts, a null central intensity, and the ability to convey internal optical orbital angular momentum (OAM).
An important characteristic of these beams is an azimuthal phase dependence eiχφ where φ is the azimuthal angle about the beam, and χ represents the overall topological charge of the internal Orbital Angular Momentum (OAM) of the beam. As a consequence of this helical phase, the phase front has a screw dislocation in the center of the beam. E fields cancel at this point giving rise to so called “donut modes” with zero intensity centers and singular phase.
One of the specific advantages possessed by OV beams for use in communications is that they can be described as the superposition of infinite set of mutually orthogonal propagating modes that are independent of polarization and wavelength. As these orthogonal OAM modes are parameterized by the distinct integer “topological charge” that can vary without theoretical limit, exploiting OAM can multiply the number of available channels in optical communications multiplexing above and beyond what is available with wavelength and polarization multiplexing.
Technology for connecting external photonic signals, free space or fiber-guided light beams, to a Photonic IC (PIC) is not quite as straightforward as connecting external electronic signals carried by copper PCB traces to ordinary electronic ICs. Existing fiber-to-PIC interface options include lens focusing, end-butt fiber coupling, prism couplers, tapered couplers, and grating (“Bragg”) couplers. Bragg couplers couple light near vertically in and out of the optical slab or film waveguides of integrated optics. The near vertical coupling of a grating coupler provides physical flexibility in placing the optical interface anywhere on the chip surface. As such, a grating coupler represents an analogous optical bond pad on the PIC where a fiber conveniently could be butt-coupled, or a free-space beam could be focused. This allows increasing the density of optical interfaces and preserves the chip edge to be used for low- and high-speed electronic signal pads.
However, there remains a continued need in the art for more efficient and adaptable devices and systems for coupling light into and out of photonic integrated circuits.